1. Field of the Invention
The present invention relates to a multiplex transmission apparatus which comprises a plurality of nodes connected to communication lines for transmission and reception of various kinds of control information in a multiplexed fashion, to/from the communication lines.
2. Description of Related Art
Recent increase of electrical components for vehicles and control systems and development of electronic control to such components and control systems have brought an enormous increase in the number of wiring, connectors and electronic control devices arranged in a vehicle body, making a wire harness attached to the vehicle body larger in size and heavier in weight. The growth of the wire harness limits attaching space in the vehicle body and degraded flexibility of wire harness lowers attaching efficiency. Further, an increase of the weight of wire harness is directly related to increase of weight of the vehicle body.
For the purpose of decreasing the number of wiring, connectors and electronic control devices, it is proposed to construct a multiplex communication system for vehicle which comprises a common communication line for multiplex transmission of plural kinds of control information to operate various electrical components and control systems in a vehicle. The common communication line is connected to a plurality of nodes which perform transmission and reception of control information to/from the line, and various electrical components and control systems are appropriately connected to the nodes.
In this multiplex communication system for a vehicle, the communication line for transmission of multiplexed control information is formed by a signal transmission path comprised of a wire harness arranged in a vehicle body. The plurality of nodes connected to the communication line are respectively combined with signal generating devices such as sensors and switches, and an operation unit such as various actuators and motors arranged in the vehicle. Each node generates control information based on a signal from the connected various signal generating devices and transmits the control information to the communication lines. Furthermore, each node receives selected control information from the communication line and generates an operation control signal based on the received control information. The node supplies the operation control signal to the operation unit to perform an operation control. The multiplexed control information transmitted through the communication line is expressed by, e.g., a pulse code modulation (PCM) signal in a form of non-return-to-zero (NRZ) modulated carrier wave signal.
In the above-mentioned multiplex communication system, upon generating control information based on a signal from a signal generating device in the connected node, or generating an operation control signal based on control information received from the communication line, in accordance with the signal from the signal generating device or the control information received from the communication line, the signal generating device or the operation unit usually requires an operation processing by an operation control unit comprising, e.g., a microcomputer. Each node connected to the signal generating device or the operation unit includes a transceiver connected to the communication line, an operation control unit connected to the transceiver, comprising, e.g., a microcomputer, and an input-output processor connected to the operation control unit and either the signal generating device or the operation unit.
As described above, in the multiplex communication system comprised of a plurality of nodes which transmit and receive control information to/from the communication line in a multiplexed fashion, the control information can be categorized into two types: periodic control information which is generated at regular intervals; and non-periodic control information which is generated at irregular intervals.
In the conventional multiplex communication system, periodic control information and non-periodic control information are multiplexed through the same communication line. In this case, if non-periodic control information is suddenly supplied to the same communication line where periodic control information is sequentially transmitted, the transmission state of the periodic control information is effected by an intrusion of the non-periodic control information, and the interval of the periodic control information may be undesirably changed.
More particularly, as shown in FIG. 13, when signals from nodes A and B exist in the same communication line at the timing t.sub.1, the node B delays its transmission of signal until the node A finishes transmitting a signal (timing t.sub.2). As shown in FIG. 14, when the nodes A and B transmit signals simultaneously, a signal having higher priority (the signal from the node A has higher priority in this embodiment) is transmitted.
When the above-mentioned undesirable change occurs, in the node where an operation control signal based on the received periodic control information is generated, and the operation control signal is supplied to the connected operation unit so as to perform an operation control of the operation unit, there may be a case where the operation control signal is not properly supplied to the operation unit.